The present invention relates to improvements in bi-fuel engines that alternatively use gasoline or gaseous fuels, and particularly to enhanced starting performance, i.e. decrease in engine start times for gaseous fuel starts that follow gasoline engine operation, and improved start consistency for gaseous fuel starts that follow gaseous fuel engine operation.
In a bi-fuel engine utilizing the combination of a) a gaseous fuel throttle body induction fueling strategy for delivery of gaseous fuel, such as compressed natural gas, and b) port fuel injection for delivery of gasoline fuels, certain shortcomings plague normal performance of the gaseous fuel operational mode. In particular, long starting times are generally associated with gaseous fuel starts following gasoline engine operation. The long starting times are attributable to the delivery of gaseous fuels upstream of the throttle body. In such cases there is a considerable delay (i.e. as much as three to four seconds) associated with engine starting times as a result of normal time for transport of gaseous fuel from fuel metering devices to the engine cylinders.
Moreover, inconsistent start quality has been associated with gaseous fuel starts following gaseous fuel engine runs. When the engine is shut down following gaseous fuel operation, a significant quantity of residual, unburned fuel may remain trapped in the intake system between the point of fuel introduction and the engine. While the engine is off, the gaseous fuel slowly bleeds out of the intake system. The quality and consistency of subsequent starts can vary considerably, depending on how long the engine has been off; i.e. how much of the gaseous fuel has bled out of the intake system.
One major drawback of prior art systems has been a lack of control of amounts of gasoline used to reduce gaseous fuel starting times. In some bi-fuel engine systems wherein gasoline has been used during starting, the starting has been difficult to control at best, as such systems have not been programmed to include robust pulse width control. In addition, such systems have lacked accommodation for soak times.
This invention provides a gasoline assisted gaseous fuel start strategy or method to address the aforementioned issues by providing a calibratable quantity of supplemental gasoline to be delivered during gaseous fuel engine starting and initial engine run. In one preferred form, the method employs a series of calibration tables that address and facilitate control of the following parameters: start of supplemental gasoline fueling, end of supplemental gasoline fueling, ramp-in period of supplemental gasoline fueling, ramp-out period of supplemental gasoline fueling, and the fraction of supplemental gasoline fuel to be delivered relative to the quantity of gasoline that would normally be delivered during a conventional gasoline start.
The method also involves differentiation between hot and cold starts for determining how much supplemental gasoline is to be delivered during gaseous fuel starts. As defined in this invention, a xe2x80x9ccold startxe2x80x9d refers to a start that occurs after diffusion of residual gaseous fuel in the intake system is deemed to be substantially complete, while a xe2x80x9chot startxe2x80x9d refers to a start that occurs before substantially complete diffusion of residual gaseous fuel in the intake system. The time required for substantially complete diffusion of residual gas depends on the geometry of any given intake system. The gasoline assisted gaseous fuel start strategy of this invention provides a means to modify (i.e. calibrate) the criteria used for hot and cold start determination so that the method can be readily applied to different vehicles.
Another feature embodied by the method is actual calibration of the quantity of supplemental gasoline fueling based on the fuel type used for the previous engine operating cycle. This is important because for gaseous fuel starts that follow gasoline engine operations, the intake system does not contain any significant quantity of residual fuel. For gaseous fuel starts following gaseous fuel engine operation, the intake system may contain a considerable quantity of residual fuel depending on how long the engine has been shut down.
Use of the gasoline assisted gaseous fuel start strategy of this invention makes it possible to sufficiently enrich the air-fuel ratio in the intake system, over a wide range of conditions, to provide consistent gaseous fuel start performance. Application of the strategy to a compressed natural gas (CNG) bi-fuel vehicle demonstrated reduced starting times, and improved consistency of gaseous fuel starts. For specific instances where CNG bi-fuel vehicles were started on CNG following gasoline operations, start times were reduced from 3-4 seconds to about 0.5 seconds under the strategy of this invention.